An inverter is an electrical circuit that converts direct current (DC) to alternating current (AC). Inverters are used in a wide range of applications, from small switching power supplies in computers, to large electric utility applications that transport bulk power. A rectifier is an electrical circuit used to convert AC into DC current. The output of the rectifier is essentially half-AC current, which is then filtered into DC. For the purpose of the present specification, inverters and rectifiers will be referred to as converters when both devices are being referred to.
Converters are known to cause harmonics in their environments, such as within the supply network or in sensitive equipment connected to the same line. To limit the harmonics that can be induced into a system, various norms have been established, such as the IEC 1000-3-2 and the EN61000-3-2 standards. In order to meet these standards, two techniques are known to reduce harmonics. The first technique is to apply filters that block the harmonics. This solution is costly and cumbersome. The second technique is the use of multilevel converters.
Multilevel converters reduce harmonics by providing an AC waveform that exhibits multiple steps at several voltage levels. The closer the waveform comes to a perfect sine wave, the less likely it is that harmonics be present. Known topologies for multilevel converters are the Neutral Point Diode Clamped Multilevel Converters, the Flying Capacitor Multilevel Converters, and the Cascaded H-Bridge Multilevel Converters. While all three of these topologies are successful in reducing harmonics, they quickly become bulky and constricting when the number of levels exceeds three, due to the large number of both active and passive components present in the circuits.
Therefore, there is a need to reduce the number of components required in a multilevel converter, while maintaining a high efficiency and generating waveforms of high quality.